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41. Stratigraphy and Sediment Accumulation Patterns of the Upper Cenozoic Pelagic Carbonate Caps of Guyots in the Northwestern Pacific Ocean1

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41. Stratigraphy and Sediment Accumulation Patterns of the Upper Cenozoic Pelagic Carbonate Caps of Guyots in the Northwestern Pacific Ocean1 Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 144 41. STRATIGRAPHY AND SEDIMENT ACCUMULATION PATTERNS OF THE UPPER CENOZOIC PELAGIC CARBONATE CAPS OF GUYOTS IN THE NORTHWESTERN PACIFIC OCEAN1 David K. Watkins,2 Paul N. Pearson,3 Elisabetta Erba,4 Frank R. Rack,5 Isabella Premoli Silva,4 Horst W. Bohrmann,6 Julianna Fenner,7 and Peter R.N. Hobbs8 ABSTRACT Many of the guyots of the Northwestern Pacific Ocean are capped by sequences of uncemented pelagic carbonate. Exami- nation of three of these pelagic caps from Limalok, Lo-En, and Wodejebato guyots in the Marshall Islands indicates that sedi- ment accumulation throughout the late Paleogene and late Cenozoic was characterized by episodes of deposition during the earliest Miocene, late early to early middle Miocene, late middle Miocene, mid-Pliocene, and Quaternary, separated by periods of little or no accumulation. The Miocene record consists of extensively winnowed foraminifer oozes, which suggest relatively energetic currents in the intermediate waters. The Pliocene and Quaternary sequences contain finer grained nannofossil oozes, suggesting lower intermediate-water current velocities. Comparison with guyots from more northern locations indicates a highly predictable relationship between modern latitude and pelagic cap thickness, with thickness decreasing toward the north. Stratigraphic and seismic reflection evidence suggests that erosion is the major factor contributing to the diminution and even- tual destruction of the pelagic caps as they are rafted northward by the motion of the Pacific Plate. INTRODUCTION and synthesis, this volume) and Premoli Silva, Haggerty, Rack, et al. (1993). The time scale used is from Cande and Kent (1992). Ocean Drilling Program (ODP) Leg 144 recovered Oligocene Here we synthesize these data to derive a coherent history for the through Pleistocene pelagic carbonate oozes from four guyots in the development of the Leg 144 pelagic caps. These data are then com- northwestern Pacific Ocean. These guyots share many characteristics bined with information from other guyots in an attempt to explain the with other guyots in the worlcTs oceans. They consist of a volcanic distribution and late Cenozoic history of pelagic caps on guyots in the edifice whose summit is often flat-topped owing to subaerial or sub- northwestern Pacific. marine erosion. Many have a shallow-water carbonate sequence atop the summit plateau. This formed early in its subsidence history, when the guyot was near sea level. Finally, many are capped by a sequence LIMALOK GUYOT (SITE 871) of pelagic sediments (referred to in this paper as the "pelagic cap"), which accumulated as the guyot subsided into the pelagic realm. Site 871 is located at 5°33.43'N, 172°20.66'E, near the summit of Limalok and Wodejebato guyots possess all three elements. Lo-En Limalok Guyot in 1255 m water depth. Limalok Guyot (formerly Guyot lacks the carbonate platform complex in the vicinity where Harrie Guyot) is in the Ratak Chain of the southern Marshall Islands Site 872 was drilled by Leg 144. MIT Guyot lacks a true pelagic cap, (see site map preceding the title page). Limalok Guyot occupies the as pelagic carbonate is present only in sheltered depressions on the same volcanic pedestal as Mili Atoll. The shallow-water carbonate summit plateau. complex on Mili Atoll continues to thrive. However, shallow-water Data for this paper are derived from examinations of several fossil carbonate deposition on Limalok Guyot ceased during the middle groups, including planktonic foraminifers (Pearson, this volume), Eocene (Nicora et al., this volume). Platform drowning was followed calcareous nannofossils (Watkins et al., this volume; D.K. Watkins by deposition of manganese-encrusted hardgrounds during the mid- and E. Erba, unpubl. data), and diatoms (Fenner, this volume). Geo- dle Eocene through (?)Oligocene (Watkins et al., this volume). Dep- technical and eolian data are derived from Rack et al. (data reports osition of the carbonate pelagic cap began during the early early Mi- ocene. Holes 871A and 87IB were drilled to recover the pelagic cap on Limalok. Hole 871A cored 151.9 m with a recovery of approximately 83%. Hole 871B had a similar penetration (152.4 m) but significantly 1 Haggerty, J.A., Premoli Silva, I., Rack, F., and McNutt, M.K. (Eds.), 1995. Proc. ODP, Sci. Results, 144: College Station, TX (Ocean Drilling Program). lower (68.2%) recovery. Given this comparative recovery, paleonto- 2 Department of Geology, University of Nebraska, Lincoln, Nebraska 68588-0340, logical investigations were concentrated on the more complete record U.S.A. from Hole 871 A. 3 Department of Earth Sciences, University of Cambridge, Downing Street, Cam- The pelagic carbonate sequence in Hole 871A consists of a single bridge, CB2 3EQ, United Kingdom. (Present address: Department of Geology, Wills Memorial Building, University of Bristol, Queens Road, Bristol BS8 1RJ, United King- lithologic unit divisible into two subunits (IA and IB) based on its dom.) lithologic and geotechnical properties. Subunit IA (Core 144-871A- 4 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via L. 1H to Section 144-871A-3H-CC; 0-26.6 m below seafloor [mbsf]) Mangiagalli 34,1-20133 Milano, Italy. consists of a light gray (5Y 6/2) nannofossil foraminifer ooze. Calci- Ocean Mapping Group, Department of Geodesy and Geomatics Engineering, Uni- versity of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada. um carbonate content is generally between 94% and 98% of the ooze. 6 GEOMAR, Forschungszentrum für Marine Geowissenschaften, Waitzstrasse 67, This subunit is characterized by generally lower porosity (70%- D-24105 Kiel, Federal Republic of Germany. 75%), lower water content (40%-50%), and higher dry bulk density 7 Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 510153, D-300 (0.75-0.9 g/cm3) than the underlying Subunit IB. The eolian compo- Hannover 51, Federal Republic of Germany. British Geological Survey, Engineering Geology and Geophysics Group, Key- nent (<63µm) makes up approximately 0.2%-0.75% of the dry sedi- worth, Nottingham NG12 5GG, United Kingdom. ment. Subunit IB (Cores 144-871A-4H to -15H; 26.5-139.5 mbsf) 675 D.K. WATKINS ET AL. consists of white foraminifer ooze. This subunit is dominated by The winnowed, waterlogged nature of the foraminifer oozes in planktonic foraminifers that generally make up more than 90% of the Subunit IB significantly affected the biostratigraphic integrity of the sediment. The medium sand texture of the sediment and its homoge- recovered sediment. Calcareous nannofossils are generally rare and nous nature indicates that it was extensively winnowed during its poorly preserved, suggesting that most were removed or at least re- deposition. The sediment in this subunit was very soupy when recov- mobilized several times during deposition. In addition, the high water ered from the hole, and considerable mixing occurred within the core content and medium sand texture of the sediment often allowed sus- liner. The winnowed nature of the sediment and its waterlogged con- pended nannofossils to move throughout much of the length of the dition are reflected in the higher porosity (75%-82%), lower dry bulk core before its curation. As a result, reworking and mixed assemblag- density (0.4-0.6 g/cm3), and higher water content (40%-50%) in es are common throughout most of Subunit IB. Given these prob- Subunit IB as compared with Subunit IA. lems, the planktonic foraminifer age determinations for this subunit Biostratigraphic analysis based on combined planktonic foramin- are thought to be more accurate in general. ifer and calcareous nannofossil age determinations indicates the pres- Two biostratigraphic intervals are recognizable in Subunit IB. ence of five intervals of temporally contiguous sediment separated by The upper interval (Core 144-871A-4H through Sample 144-871A- four disconformities (Fig. 1). Three of these sediment units occur 6H-6, 60-61 cm; 25.7-54.2 mbsf) contains planktonic foraminifer within lithologic Subunit IA. assemblages of middle Miocene Zone N12. These sediments proba- The youngest interval includes sediment from Core 144-871 A-1H bly contributed the Fohsellafohsi group specimens reworked into the through Sample 144-871A-3H-2, 60-61 cm (0-19.1 mbsf). This in- base of Subunit IA. Calcareous nannofossils in this interval contain terval is contained within planktonic foraminifer Zone N22. Calcare- poorly preserved assemblages attributed to Zone CN5. Preservation ous nannofossil biostratigraphy indicates a relatively complete se- in the base of this interval is too poor to allow subzonal assignment. quence of sediments. All seven of Gartner's (1977) Pleistocene zones Better preserved assemblages in the upper part of the interval contain are represented. In addition, the base of this sequence contains nan- identifiable Discoaster kugleri, indicating Subzone CN5b. This sub- nofossil assemblages of Subzone CN12d of latest Pliocene age. The zone is, in part, correlative with planktonic foraminifer Zone N12. last appearance datum (LAD) of Globigerinoides fistulosus and the Sediment accumulation in this interval was more rapid (approximate- first appearance datum (FAD) of Gephyrocapsa oceanica, criteria for ly 17 m/m.y.) than that in Subunit IA. This interval is separated from delimiting the Pliocene/Pleistocene boundary for foraminifers and the underlying one by a disconformity with a hiatus of at least 1.3 nannofossils, respectively, both occur at the base of Core 144-871A- m.y. 2H. The overall rate of sediment accumulation for this interval is ap- The lower interval (Section 144-871A-6H-CC through Core 144- proximately 10 m/m.y. Examination of the sediment accumulation 871A-15H; 55.0-139.2 mbsf) in Subunit IB consists of a relatively rates for individual nannofossil zones indicates that most are within continuous sequence of lower to lower middle Miocene foraminifer the range of 8-14 m/m.y. The sediment accumulation rate within the ooze (Fig.
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